Controlled grain size casting method



United States Patent 0 3 158,912 (IGNTRGLLED GRAThl SlZil CASTENG li iETHGD Wilhur H. Schweiliert, (Iincinnati, @hio, assigaor to Seneral Electric Qcmpany, a corporation of New York No Drawmg. Fihl Aug. 9, I962, Ser. No. MEI/75 3 Claims. (6i. 22-=-t%} This is a continuation in part of copending application Serial No. 811,788, filed May 8, 1959, now abandoned, and assigned to the assignee of the present application.

This invention relates to metal casting and, more particularly, to a precision casting method, slurry and mold for providing nuclei to promote rapid solidification of metal cast into a mold and to produce a fine grained structure.

Included with the factors of chemical composition and heat treatment determining strength properties of cast metal alloys is the factor of size of the alloys grain structure. Thus grain size limits are specified in the manufacture of many articles for use under high stress con ditions especially at elevated temperatures.

One of the physical properties of a material improved by a fine grain structure is its fatigue life. Fatigue life may be defined as the time material in an article will resist breaking under specific conditions of repeated cyclic stressing.

Methods known to produce fine grained structures in cast materials involve provision in the molten metal of nuclei around which metal grains start to grow upon cooling after casting. Such nucleation has been brought about by methods such as dispersing particles in the melt after pouring, pouring metal in a partially solidified or slushy condition, or cycling the temperature of the melting furnace to freeze out nuclei just before pouring. In each case, the size of the grains produced depend on the number of nucleation centers which depend in part on the melting and pouring conditions. However, in many cases, conditions such as temperature are difficult to control with a high degree of precision. A variation in grain size may result from article to article thus varying the strength properties and reducing uniformity among articles.

A principal object of my invention is to provide, in a method for casting articles, an accurate, reproducible method for producing a uniform, fine grained structure.

Another object is to provide, in precision casting techniques, a method for providing a controlled number of nucleation centers available on the inner lining of a mold to produce controlled grain size throughout a precision cast article.

Still another object is to provide a slurry and a mold lining portion made from such slurry which provides a controlled number of nucleation centers on the mold lining surface for grain size control.

These and other objects will become more apparent from our description taken in conjunction with the appended claims.

My invention, in one form, includes the steps of providing a slurry from which an investment casting mold can be made, the slurry consisting essentially of about 15% by volume of a metal compound reducible to a free metal at a temperature no higher than the pouring temperature of the casting metal. The metal element of the reducible metal compound is one of the group of metals iron, cobalt, nickel, and manganese, the balance of the slurry being mold material. The method aspect of the invention includes reducing the metal compound to a free metal on the lining of the mold so that the reduced metal in a finely divided state will act as nucleation centers for metal grains formed on cooling the subsequently cast metal.

In another form, my invention includes the provision "ice of an investment casting mold into which casting metal is poured having a lining portion consisting essentially of about 1-5 by volume of the reducible metal compound of the above described slurry.

The general procedure of one of the precision casting methods to which my invention is applicable is sometimes referred to as the lost wax method or investment casting. This method, which I will use as an example of the application of my method, includes the formation in an accurate master metal mold of an expendable pattern of a material which can be melted, vaporized or dissolved away, for example of a plastic, wax, or a low melting metal. Such pattern accurately reproduces the contours of the master mold. A number of these expandable patterns may then be grouped together. The patterns are then covered or coated with a dip coat or generally thin layer of non-expendable material sometimes called primary investment. The dip coat is frequently in the form of a powder slurry to assure a fine, smooth surface finish. The coated single or grouped patterns are assembled with appropriate gates and risers of expendable material within a mold shell. The mold shell is filled with back-up mold materials sometimes called investment slurry or secondary investment which is allowed to harden or cure. The expendable materials are then removed from the hardened mold such as by heating to melt or vaporize the expendable material.

Thus a mold cavity is created having smooth surface or liner of the non-expendable dip coat material or primary investment originally applied to the surface of the expendable material. Into this prepared mold cavity, molten metal is poured which metal, upon cooling, retains the shape of the mold cavity and reproduces the surface finish on the mold liner.

A modification of the above described precision casting process to which my invention may be applied is sometimes called the single investment or single shell mold type process. In this modification, the above de scribed generally thin dip coat is replaced by a directly applied single investment material which, after removal of the expandable material, becomes the complete mold without back-up material or secondary investment. It is into this single shell without back-up that the metal to be cast is poured. Such a single investment mold may actually be made up of a series of laminations to provide strength to the mold. One method used for producing such a shell involves the application of alternate layers of dry powdered refractory ceramic or glass to the wet slurry layer applied around the expendable pattern with drying or setting periods between laminations. Thus a shell is build up which can later be fired to harden, join more tightly or fuse together the laminations.

In connection with the above described casting methods, my invention relates to the addition to the inner mold surface of dip coat material of a metal compound reducible to a free metal at or below the casting metal pouring temperature in the lost wax type process or to the inner suface in the single investment type process. Such added material can be included in the wet slurry in a quantity from about 1 up to about 5 percent by volume of the slurry to provide a lining portion to be contacted by molten metal. The type and amount of material added depends on the type of dip coat material or the single investment material, many varieties of which are in use in industry.

In practicing my invention of providing a casting mold lining including a metal compound conductive to reduction to a free metal at or below the temperature of the metal when poured, I include as added materials the compounds of such metals as iron, cobalt, nickel, and

7 reduction of their oxides to free metal.

'init's free state or'in the" form of'a reducible compound in'the mold should be determined, for example by varying the 'm'etal'compound content ofthe dip coat or single investment material between the quantity ofyaboutlj up to about 5 'p'erc'entby' volume of the dip coat or single investment'ni'at'erial in its wet orslurry form. A-mold lining made from such a slurry includes material capable of being'reduced to act as initial nucleating triggers.

Designers of such apparatus as jet engines, with the assistance of supporting metallurgists, have found that the strength characteristics of turbine blades can greatly be improved by control of grain size; Normal precision casting techniques using'high temperature metals ofth'e type listedin the following table haveiresulted in metal grains in castings havingdiam'eters'of about Ai- /s" size or greater whereas it is desirable to limit the diameter of grains to /8 or less, preferably in the range.

the mold provides additional heat to bring about reduction. For an instant after reduction, and in many cases only a microsecond, the added material becomes a great number of molten globules, for example of primary iron. However, since the mold is' customarily at a temperature below the melting point ofthe pure metal in the globules, the globules immediately freeze thus creating a large number of nuclei around which molten metalor alloy being cast starts to freeze in alarge number of places. Thus a fine grain structure is producedwhich is propagated almost irnmediately'throughout the castingso that there are no areas which remain molten for a time suffi cient to allowlarger grains to grow. I

Exa mp le 'l Thepre'viously described lost wax-process waspracticed in a normal manner without the inclusion of a reducible material in the dip coat and without injection of a reducing atmosphere into the mold prior to pouring/the casting metal. The mold was pro-heated and maintained at about 1800 F. prior-toyacuumcasting, of themetal. The casting metal used had the nominal composition in percent by weight of 0.08 max. carbon, 0.1 max; manga nese, 0.15 max. silicon, l5 chromium, 3.5 titanium, 0.08 boron, 4.6 aluminum, 5.25'molybdenum, 5 iron,.wi=th'the balance essentially nickel. This alloy was poured at'a TABLE Nominal Composition (Wt. percent) Preferred Reducible 1 Alloy Name metal Cr Ti Al Mo B Zr W gb/ Cu Mn Si 0 ;Fe 00 N1 (oxide) I havefound-that by making a mold lining portion from anormal, commercially available mold slurry to which was added about 1-5 by volume or" a reducible metalcompound which can be reduced at or-below 'the casting metal pouring temperature, thedesired size grains can bedeveloped; Howeven'as'shown by the examples, the inclusion of such material below about /2% by volume and above about 10% by volume had no effect on reducingthe grain size under the same-conditions. In addition, I havefound that above about 10%* by vo ume, the mold lining-portion-has atendency to break down thus to result in unacceptable'surface finishes on the 'castarticler f Whenmy method-is employed, thevery surface of the cast article is-the only area which exhibits any change in chemical composition 'due' to' added material. Such change, however, amounts only toa fractionof a percent and does not afi'ect the general chemical composition more than a few thousandthsof aninch-from'the-surface. In addition, th re isreasonto believe that such a surfacev layer, for example, anironrichsur-face layer, can be beneficiatfroma fatigue standpoint on castings used in the as-cast condition. a

The above described type of reducible compound, such as an oxide, located Orr-the mold lining is reducedin' my process by the injectiondnto-the moldcavity just before metal is poured into the mold of-a material which will producea reducingatmosphere. An example of such a material with which Ihavehad success is trichlorethylene and heptane, although aninjection can be made-of any material which will bring about'a rapid reduction of the added material. 'j g If the temperatureto which the mold is' heated prior to pouring-of the casting metal is notsuflicient to allow the reducing atmosphere to'reduce the addedrnatcrial to a free metal, then the casting metal when poured into temperature of about 2850 35., a temperature of about 450 F. superheat over the casting alloys melting point to avoid piping and provide molten metal for back fill during solidification. The size of grains of'thearticle" resulting from this casting-wasin excess of one-quarter inch; the article exhibited relatively poor fatigue life.

Examples 2- 5 The procedure of Example 1 was repeatedrusing the same materials except that a seriesiof standard dip coats to which-the addition separately of 1, 3, 5 and 10 percent by volume of very finely powderedFe O were substi tuted for the standard dip coat inExample 1. Trichlorethylene was injected into the mold cavity just prior to pouring of the casting metal'th'us to create areducing atmosphere capable of'reducing'the' FegO at a tempera ture of2850 F. or below. The articles cast from the molds prepared with dipcoats including. the 1, '3 and 5 percent by volume 1% 0 had grains inthe /32" /8" grain size range and exhibited relatively good" fatigue life; However, the. article cast in a mold made with the: 10 percent byvolume additionot Fe 0 produced grains in excess of A1" size. i V

Thus the dip coat including between1-5 percent by volume Fe O resulted-in a finer grained casting of improvedfatigue life.

Example v6 C00 to the standard dip coat'resulted in an improved castingot finer grain structure.

Example 7 7 The procedure of Example 6 was repeated with'the U same results except that MnO was substituted for the (300.

Although I have described my invention in connection with specific examples, it will be understood by those skilled in the art, the modificadons and variations of which my invention is capable.

What is claimed is:

1. In a method of casting an article in an investment casting mold, the steps of: producing a lining portion of an investment casting mold having a lining surface to confine molten metal from a slurry consisting essen" 13/ of about 1-5% by volume or" a metal compound reducible to a free metal at a temperature no higher than the youring temperature of the casting metal, the metal el ment of the reducible metal compound being selected from the group consisting of iron, cobalt, nickel, and manganese, the balance of the slurry being mold material; and then, at the time of pouring, reducing the metal compound on the lining surface of t e mold to free metal in a finely divided, discrete state whereby the free metal is available as metallic nuclei for the formation of fine metal grains on contact with the molten metal subsequently cast into the mold.

2.,In a method of precision investment casting, the steps of: producing around an expendable pattern a nonexpendable investment cast'mg mold lining portion from a slurry consisting essentially of about 15% by volume of an iron oxide reducible to free iron at a temperature no higher than the pouring temperature of the casting metal, the balance of the slurry being non-expendable investment casting mold material; removing the expendable pattern from within the non-expendable mold lining portion at the inter-face with the exeendable pattern becoming a lining surface of the mold to confine molten metau; and then, at the time or" pouring, reducing the iron oxide on the surface of the mold to free iron in a finely di"'ded, discrete state whereby the free iron is available as metallic nuclei or the formation of the metal grains on contact with the molten metal subsequently cast into the mold,

3. in a method of precision investment casting, the steps or": producing around an expendable pattern a nonexpendable investment casting mold lining portion from a slurry consisting essenually or" about l5% by volume or a cobalt oxide reducible to free cobalt at a temperature no higher than the pouring temperature of the casting metal, the balance of the slurry being non-ex endable investment casting mold material; removing the expendable nattern from Within the non-expendable mold lining portion, the surface of non-expendable mold lining portion at the interface With the expendable pattern becoming a lining surface of the mold to confine molten metal; and then, at the time of pouring, reducing the cobalt oxide on the lining surface of the mold to free cobalt in a finely divided, discrete state whereby the free cobalt is available as metallic nuclei for the formation of fine metal grains on contact with the molten metal subsequently cast into the mold.

References Qited in the file of this patent UNlTED STATES PATENTS 2,592,337 Robertson et a1. Aug. 8, 1952 3,019,497 Horton Feb. 6, 1962 FOREIGN PATENTS 755,073 France Sept. 4, 1933 OTHER REFERENCES Shell Molded Stainless Castings, article in Iron Age, June 26, 1952, pages 112-116. 

1. IN A METHOD OF CASTING AN ARTICLE IN AN INVESTMENT CASTING MOLD, THE STEPS OF: PRODUCING A LINING PORTION OF AN INVESTMENT CASTING MOLD HAVING A LINING SURFACE TO CONFINE MOLTEN METAL FROM A SLURRY CONSISTING ESSENTIALLY OF ABOUT 1-5% BY VOLUME OF A METAL COMPOUND REDUCIBLE TO A FREE METAL AT A TEMPERATURE NO HIGHER THAN THE POURING TEMPERATURE OF THE CASTING METAL, THE METAL ELEMENT OF THE REDUCIBLE METAL COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OF IRON, COLBALT, NICKEL, AND MANGANESE, THE BALANCE OF THE SLURRY BEING MOLD MATERIAL; AND THEN, AT THE TIME OF POURING, REDUCING THE METAL COMPOUND ON THE LINING SURFACE OF THE MOLD TO FREE METAL IN A FINELY DIVIDED, DISCRETE STATE WHEREBY THE FREE METAL IS AVAILABLE AS METALLIC NUCLEI FOR THE FORMATION OF FINE METAL GRAINS ON CONTACT WITH THE MOLTEN METAL SUBSEQUENTLY CAST INTO THE MOLD. 